Wireless communication systems, such as the 3rd Generation (3G) of mobile telephone standards and technology, are well known. An example of such 3G standards and technology is the Universal Mobile Telecommunications System (UMTS), developed by the 3rd Generation Partnership Project (3GPP) (www.3qpp.orq).
Typically, wireless communication units, or User Equipment (UE) as they are often referred to in 3G parlance, communicate with a Core Network (CN) of the 3G wireless communication system via a Radio Network Subsystem (RNS). A wireless communication system typically comprises a plurality of radio network subsystems, each radio network subsystem comprising one or more cells to which UEs may attach, and thereby connect to the network.
The 3rd generation of wireless communications has been developed for macro-cell mobile phone communications. Such macro cells utilise high power base stations (NodeBs in 3GPP parlance) to communicate with UEs within a relatively large coverage area.
Lower power (and therefore smaller coverage area) femto cells or pico-cells are a recent development within the field of wireless cellular communication systems. Femto cells or pico-cells (with the term femto cells being used hereafter to encompass pico-cells or similar) are effectively communication coverage areas supported by low power base stations (otherwise referred to as Access Points (APs)). These femto cells are intended to be able to be piggy-backed onto the more widely used macro-cellular network and support communications to UEs in a restricted, for example ‘in-building’, environment.
In this regard, a femto cell that is intended to support communications according to the 3GPP standard will hereinafter be referred to as a 3G femto cell. Similarly, an access controller intended to support communications with a low power base station in a 3G femto cell will hereinafter be referred to as a 3rd generation access controller (3G AC). Similarly, an Access Point intended to support communications in a femto cell according to the 3GPP standard will hereinafter be referred to as a 3rd Generation Access Point (3G AP).
Typical applications for such 3G APs include, by way of example, residential and commercial (e.g. office) locations, ‘hotspots’, etc, whereby an AP can be connected to a core network via, for example, the Internet using a broadband connection or the like. In this manner, femto cells can be provided in a simple, scalable deployment in specific in-building locations where, for example, network congestion at the macro-cell level may be problematic.
As will be appreciated by a skilled artisan, in a large scale deployment, there can be as many as a few million femto cells interspersed within the macro cellular layer. In a co-channel deployment in particular, interference from femto cells becomes a major problem for macro cell receivers. Noise experienced by the macro cell receivers, caused by interference from femto APs and wireless communication units within femto cells, leads to a reduction in the effective coverage area for that macro cell.
As is known, within each cell of, say, a UMTS network, a Common Pilot CHannel (CPICH) is broadcast by the base station (e.g. Node B or femto AP) for that cell. The CPICH comprises a known bit sequence that can be discovered by User Equipment (UE) within the cell as a phase reference for other channels and to obtain measurements for that cell.
Typically, a CPICH is broadcast with a transmit power of around 10% of the total available transmit power for that cell. The transmit powers of other common control channels and HS-PDSCHs are normally set to be proportional to CPICH transmit power. As a result, if CPICH transmit power of a femto cell can be properly controlled to a lower level, the interference experienced by neighbouring cells, such as over-lapping macro cells, can be significantly reduced. However, as previously mentioned, the CPICH is used by UEs as a phase reference for other channels and to obtain measurements for that cell. Consequently, the transmit power of the CPICH directly affects the coverage of a cell. Accordingly, whilst reducing the transmit power of the CPICH may reduce the interference caused by that cell to neighbouring cells, it also reduces the effective coverage area of the cell.
For example, networked controlled cell breathing is a known load-handling scheme based on dynamic cell coverage adjustment implemented within macro cells. When a macro cell becomes heavily loaded, it becomes more difficult to maintain a particular quality of service, especially for users that are located towards the edge of the cell. Network controlled cell breathing involves changing one or more parameters, for example the CPICH transmit power or the maximum propagation loss allowed for users in the cell, in such a way as to reduce the effective cell coverage area. In this manner, user traffic at the periphery of the macro cell is forced to move to neighbouring cells that are more lightly loaded. Once the cell load drops, the cell coverage area can be increased again.
As will be appreciated by a skilled artisan, in the case of femto cells, where typically only cell subscribed users may be permitted to access services via the femto cell, cell loading may be more easily controlled by way of limiting the number of cell subscribed users, and/or enabling/disabling cell subscribed users. Thus, network controlled cell breathing schemes as used in macro networks are not appropriate for femto cells, and thus are only implemented within macro cells, where the reduction in the effective coverage area (e.g. by reducing the transmit power of physical channels, such as the CPICH) to force users to be redirected to neighbouring cells, is not problematic, and indeed is advantageous. However, this does not translate to femto cells, where management of interference to the macro network and effective coverage to cell-subscribed users are of greater importance than load handling.
Conventionally, the CPICH power of a Node B is configured according to network and cell planning, and in particular according to a required cell coverage area. However, due to the more ad hoc nature in which femto cells may be deployed, such cell planning is not feasible for femto APs. It is therefore proposed that a femto AP is able to scan for, receive, and measure transmissions from base stations, including macro cell and other femto cell base stations, in a manner that is termed Network Listen. Some recent proposals for setting the CPICH power of a femto AP comprise adaptively adjusting the CPICH power for the femto AP based on information about neighbouring cells obtained using Network Listen. Whilst such approaches may help to reduce inter-cell interference, they do not take into account user or UE information or requirements. Consequently, such approaches can have a detrimental effect on the quality of services provided to UEs.
Another problem that has been identified with femto cells is that, due to the problem of interference caused by femto cells to, for example, overlapping/neighbouring macro cells, it is desirable to keep the overall available transmission power of the femto AP to within a constrained level. As a result, femto APs typically have a limited amount of power available for use for physical channels. When a large number of users require services from the femto AP, the power available for providing each service can become significantly low, thereby affecting the quality of service that is provided to the UEs.
Thus, there exists a need for an apparatus and a method for setting transmit power levels that substantially alleviates at least some of the deficiencies with current techniques and methods therefor.